The sol-gel method is a well known process for preparing silica-like materials at low temperatures (25-80° C.), generally consisting of sol preparation, gelling the sol in a mold, drying, firing and sintering (Klein, 1988 and 1993; Hench and West, 1990). This process is implemented in various applications such as fabrication of optical elements and protective and functional coatings, ceramic powders, fabrication of optical fibers, matrices for cosmetic products and insulation. Traditional sol-gel process involves the chemistry of metal alkoxides precursors such as silicon alkoxides and similar metals alkoxides such as titanium, aluminum and zirconium, resulting in amorphous inorganic materials. A technique growing in use is fabrication of inorganic-organic hybrid sol-gel using organically modified alkoxides as precursors. These hybrid products are commonly known as organically modified silicates (ORMOSILs).
One of the drawbacks of sol-gel products prepared by the conventional sol-gel process, i.e., using the traditional alkoxides as precursors, is the formation of cracks that limits the achievable bulk size to a few centimeters, or the achievable film thickness to less than 1 μm (Dislich, 1988). In order to overcome these limitations, the introduction of an organic polymer into the sol-gel matrix was suggested (Schmidt, 1989) to thereby generate an inorganic-organic hybrid. The basic precursors for fabrication of an inorganic-organic hybrid are the organically modified alkoxides developed by Schmidt (Schmidt, 1984), i.e., alkoxides in which at least one alkoxyl group is substituted by an organic tail, which releases stress and gives the sol-gel product additional flexibility, thus allowing the material to withstand the capillary forces during the formation process and enabling the formation of large bulks or thick films (tens to hundreds μm). Various organically modified alkoxides such as methyltrimethoxysilane (MTMS), dimethoxydimethylsilane (DMDMS), methacrylatepropyltrimethoxy-silane (MATMS) and 3-glycidoxypropyltrimethoxy-silane (GLYMO) are used as precursors for the preparation of inorganic-organic hybrids (Altman et al., 1991; Pellice et al., 2006). However, in cases the majority of the precursors are inorganic alkoxides as to obtain a glass-like material, a very slow and long drying process, at up to about 600° C., is required; and in cases the majority of the precursors are organically modified alkoxides, e.g., in organic modified ceramics (ORMOCERs) (Haas et al., 1999a-b), a more organic-like polymer, i.e., a polymer containing more than 50% organic residues, is obtained. The latter suffers from the well known limitations of organic polymers, which are (i) lower transparency compared to glass; (ii) higher absorption in the UV region; (iii) larger dispersion, i.e., change in refractive index as a function of wavelength; (iv) larger thermal expansion coefficient; and (v) a larger thermo-optic coefficient (temperature dependence of the refractive index, dn/dT).
In order to accelerate the sol-gel process while still producing a crack-free product, a method for controlling the evaporation rate was introduced by Haruvy (Haruvy et al., 1992). This method, known as the fast sol-gel method, consists of semi-supercritical drying, in which the evacuation of volatile solvents is done under controlled temperature and pressure conditions. The fast sol-gel manufacturing process is very rapid (˜few minutes) and leads to a viscous sol-gel resin containing 10%-40% organic residues, combining in this way some of the advantages of organic polymers such as negligible cracks-formation and shrinkage during the process with the advantages of silica such as improved thermal stability (Gutina et al., 1999). The curing however of this resin takes usually several hours or more (Gvishi, 2009).